How much can we improve two-section watch bills? Sleepiness,Fatigue,and Psychomoter Vigilance Performance during naval operations in the 6/6 and 7-5-5-7 watch bills

Abstract

A traditional two-section watch bill (6 hours on / 6 hours off, starting at midnight – “6/6”) and a two-section watch bill better adapted to the human circadian rhythm (watches of 7, 5, 5, and 7 hours duration, starting at 3 a.m. – “7-5-5-7”) were trialed on two vessels of the German Navy with 67 study participants in a crossover design. Questionnaires were used to assess subjective sleepiness (ESS, KSS) and subjective fatigue (FSS). The psychomotor vigilance task (PVT) was self-administered with wearable devices before and after each watch. Over the course of seven days at sea, PVT performance worsened significantly with the 6/6, but not with the 7-5-5-7 watch bill. In the 6/6 watch bill, number of participants with excessive sleepiness and high levels of fatigue was higher than in the 7-5-5-7 watch bill although not at a statistically significant level. Marked differences between watch bills in crew performance started to emerge after 6 days.

Keywords

seafaring navigation sleepiness fatigue performance watch bill design

Background

The continuous operation of vessels at sea requires sailors to work in shifts covering the 24 hours of a day. The timing, duration, and assignment of these shifts – in seafaring, called watches – is spelled out in the ship’s watch bill. The debate on how to design watch bills has been ongoing for decades, with publications on empirical watch bill research dating as far back as 1949 (see Colquhoun, 1985, for an early overview). The main determinants of watch bills are (1) the number of sections, i.e., teams that take turns in the operation of the vessel, (2) the duration of watches, and (3) the timing of watches. From a human factors perspective, it seems obvious that a higher number of watch sections is preferable, because it imposes less watch duty on individual sailors and leaves more time for rest. Nevertheless, two-section watch bills are frequently employed due to a lack of personnel, inadequate space to accommodate three or four sections, or when cost efficiency is prioritized.

In a traditional two-section watch bill, 6 hours on duty are followed by 6 hours off, with turnover times at 6 a.m., 12 a.m., 6 p.m., and midnight. This schedule leaves sailors with rather short sleep durations (4 to 5 hours during the 6 hours off watch) and half of the crew gets almost no sleep during nighttime. A better adaptation of two-section watch bills to the natural human circadian rhythm (Roenneberg et al., 2007) can be achieved in two ways. Allocating a longer rest period for uninterrupted sleep to both sections results in watch bills with consecutive watches of 7, 5, 5 and 7 hours or 8-4-4-8 hours in duration (i.e., 7-5-5-7 or 8-4-4-8). The latter approach allows both watch sections to sleep during their circadian low points, which can be achieved by turnover times between the two night watches between 3 a.m. and 4 a.m. The disadvantages of the revised circadian watch bills are extended duty hours for the sailors standing watch during the extended rest period of the relieved watch, and a mismatch between turnover times and standard mealtimes, which can make eating less relaxing and meal service more complicated. Paul and Love (2022) measured sleep and wake times during naval operations in traditional 6/6 as well as 7-5-5-7 and 8-4-4-8 two-section watch bills. Based on these data, they report that models of cognitive effectiveness show over time a more pronounced performance decrement in the 6/6 routine as compared to the 7-5-5-7 or 8-4-4-8 watch bills.

In our research, we set out to directly assess and compare vigilance performance and subjective well-being in a 6/6 and a 7-5-5-7 watch bill. Our study was conducted on two mine countermeasure vessels (MCMV) of the German Navy in a crossover repeated measures design. Aims of this study were to test whether and how circadian watch turnover times can be employed during live operations, and whether the performance advantage of the 7-5-5-7 watch bill predicted by Paul and Love (2022) or advantages in subjective well-being, can be found in the field.

Materials and Methods

Participants

Sixty-seven sailors (8 female, 17 enlisted, 38 non-commissioned officers, 12 officers) between the ages of 18 and 43 (mean 29.4, SD 5.8) from two mine countermeasure vessels (MCMV, Figure 1) of the German Navy participated in this study. At the beginning of the study, all participants received written and oral descriptions of the study procedures and provided written informed consent. Approval of study procedures was obtained from the ethics committee of the University of Kiel (reference D 653/20).

Figure 1.

Mine countermeasure vessel participating in the study.

Watch Bills

Two two-section watch bills were employed on the MCMVs during the study. A traditional 6-6 watch bill starting at midnight and a revised circadian 7-5-5-7 watch bill starting at 3 a.m. (Figure 2).

Figure 2.

Watch bills on trial. Blue: On duty. Grey: Off duty. Upper watch bill: traditional 6/6. Below: revised circadian 7-5-5-7.

Length and timing of the duty hours in the revised circadian watch bill were decided with the concurrence of the commanding officers. Meals were served around watch turnover times, so the resulting watch bill is a compromise between circadian sleep times, the timing of the meals, and operational constraints.

Measures

Sleepiness

Current sleepiness was assessed daily at dinner time (between 5 and 8 p.m.) with the one-item Karolinska Sleepiness Scale (KSS, Åkersted & Gilberg, 1990), with scores ranging from 1 to 9. Average daytime sleepiness during the voyage was assessed retrospectively with the Epworth Sleepiness Scale (ESS, Johns, 1992). For ESS, the sums of all item scores, ranging from 0 to 24, were analyzed. ESS values greater than 10 have been found to indicate elevated levels of daytime sleepiness (Rosenthal & Dolan, 2008).

Fatigue

The revised version of the Fatigue Severity Scale (FSS, van Nes et al., 2009) was used to collect subjective ratings of fatigue symptoms experienced during the voyage. The sum of all items can be between 0 and 21. Values > 13 correspond to the mean value of ≥ 5 in the original FSS scale (Krupp et al., 1989), which indicates severe fatigue in clinical samples such as patients with depression or multiple sclerosis (e.g., Ferentinos et al., 2011; Möller et al., 2011).

Sleep assessment

To assess timing and duration of sailors’ sleep episodes, actigraphy data were recorded from all study participants with Motionlogger® Micro Watches (Ambulatory Monitoring, Inc.). Participants kept paper-pencil activity logs as a backup for missing actigraphy data and to gain insight into the duration of different daily activities (e.g., watch duties, administrative work, leisure time). Because the analysis of actigraphy data is ongoing, these results are not included in the current paper.

Psychomotor Vigilance Performance

Performance was determined in terms of reaction times and errors from the psychomotor vigilance task (PVT, Basner et al., 2011). The PVT is a simple reaction time task, requiring participants to observe an empty display and respond as quickly as possible to a visual stimulus by pressing a button. The PVT was self-administered by the study participants using the build-in PVT on their actigraphy devices (Figure 3). PVTs were performed four times a day at watch turnover times. The duration of the individual PVT runs was 3 minutes with the interstimulus interval (ISI) ranging from 3 to 10 seconds.

Figure 3.

Sailor performing the psychomotor vigilance task with his actigraphy device.

Data collection

Watch bills were trialed in a cross-over design (Figure 4).

Figure 4.

Days in harbor, days at sea and assignment of watch bills to the participating units over the course of the data collection.

Two MCMVs operated with the traditional watch bill and with the revised circadian watch bill for a 7-day voyage each. Unit 1 started with the revised circadian watch bill, while unit 2 used the traditional 6/6 watch bill for the first week of data collection. After two nights in port (days 8 to 10), both units adopted the remaining watch bill for their second 7-day voyage. The port stay was planned to allow all sailors to catch up on sleep they may have missed during the first voyage. For the duration of the study, the ships formed a task group operating on comparable tasks at the same time, in the same area and hence, under the same weather conditions.

Actigraphy devices were worn 24/7 throughout the trial. Exceptions were showers and activities for which occupational safety required sailors to take off watches. PVTs were performed four times a day at watch turnover times, i.e., around the start and the end of each watch. KSS ratings and activity logs for the previous 24 hours were collected after dinner each day at sea. ESS and FSS questionnaires were administered in port at the end of each voyage on days 8 and 17.

Analysis

All statistical analyses were carried out with R version 4.0.2 (R core team, 2020). Statistical significance of differences in frequencies for the ESS and FSS was determined with Lancaster’s mid-P version of Fisher’s exact test (Lancaster, 1961; Hwang and Yang, 2001) and effects of the watch bill on these dependent variables were analyzed using the aov command.

In analyzing the data of the KSS and PVT, we accounted for the multilevel structure of the data and the longitudinal design by estimating linear mixed models (LMM) using the lme4 package (Version 1.1-23) and the lmerTest package (Version 3.1-2). Simple contrasts were calculated using the emmeans package (Version 1.5.1). The models specified the fixed effects of the contrast-coded independent variables watch bill (-1 = traditional, 1 = circadian) and order of the watch bills (-1 = traditional first, 1 = circadian first) as well as for the grand-mean centered predictor voyage day and their interactions as predictors. Random intercepts as well as by-watch bill random slopes were included for participants, day on voyage, and measurement time (the latter only for PVT) to reduce the likelihood of high Type 1 error rates as a consequence of the within-subject manipulations.

Results

Sleepiness

ESS

Table 1 shows the mean ESS scores obtained from both units at the end of their one-week voyages. ESS scores tended to be lower at the end of the second voyage, and more so for unit 2 that operated with the circadian watch bill during voyage 2. Compared to the ESS standard deviations, these differences are small. No significant effect of watch bill on ESS scores was found, F(1,65) < 2.0, n.s.

Table 1.

ESS means and standard deviations.

		Voyage 1		Voyage 2
	N	M	SD	M	SD
Unit 1 (7-5-5-7 first)	35	8.9	5.3	8.5	4.0
Unit 2 (6/6 first)	32	8.8	4.9	7.8	4.6

The number of study participants with ESS scores greater then 10 is shown in table 2 for the traditional 6/6 and the circadian 7-5-5-7 watch bill. This number is lower in the circadian watch bill, but the difference is below the threshold for statistical significance (Lancaster mid-p=.14).

Table 2.

Number of participants with ESS scores greater than 10 in 6/6 and 7-5-5-7 watch bills.

	ESS ≤ 10	ESS > 10
7-5-5-7	50 (75%)	17 (25%)
6/6	42 (63%)	25 (37%)

KSS

Mean KSS values and standard deviations over the course of the one-week voyages are plotted in figure 5 for the traditional and the circadian watch bill. There was no significant effect of the watch bill on KSS scores, t(25) = 1.47, p = .155, but we found a significant effect of voyage day, t(776) = 2.15, p = .032. The interaction of watch bill and voyage day for KSS failed to reach significance, t(776) = 0.55, p = .582. Hence, KSS increased with increasing voyage duration independent of the watch bill.

Figure 5.

Means and standard deviations of KSS in the traditional (red) and the circadian watch bill (blue) over the course of the 7-day voyages.

Fatigue

Table 3 gives an overview over the mean FSS scores for the two voyages and the participating ships. On both ships, a higher mean FSS score was observed at the end of the second voyage as compared to the first voyage. This increase was more marked on ship 1, which employed the 6/6 watch bill during voyage 2. Inferential statistics found no significant main effect of watch bill on FSS scores, F(1,65) < 1.0, n.s.

Table 3.

FSS means and standard deviations.

		Voyage 1		Voyage 2
	N	M	SD	M	SD
Unit 1 (7-5-5-7 first)	35	9.5	3.8	11.0	5.5
Unit 2 (6/6 first)	32	11.1	6.5	11.5	6.7

The number of participants with markedly increased FSS scores was slightly higher in the 6/6 as compared to the 7-5-5-7 watch bill (Table 4). Again, this difference was too small to indicate a significant effect of the watch bill on the frequency of increased FSS levels (Lancaster mid-p=.46).

Table 4.

Number of participants with FSS scores greater than 13 in 6/6 and 7-5-5-7 watch bill.

	FSS ≤ 13	FSS > 13
7-5-5-7	48 (72%)	19 (28%)
6/6	44 (66%)	23 (34%)

Vigilance

Figure 6 shows means and standard deviations of PVT reaction times in the 6/6 and the 7-5-5-7 watch bill over the course of the one-week voyages. We found no main effect of watch bill (t(59) = -0.44, p = .66), but a significant main effect of voyage day (t(1739) = 2.44, p = .014) as well as a significant interaction of watch bill and voyage day, t(1744) = 1.97, p = .049. In the revised circadian watch bill, voyage day had no significant effect (t(1737) = 0.33, p = .738) on mean reaction times, with 480ms on day 1 and 485ms on day 7. In contrast, in the traditional watch bill, mean reaction times increased significantly from 449ms at day 1 to 584ms at day 7, t(1736) = 3.18, p = .002. Therefore, whereas psychomotor vigilance performance as expressed in reaction time remained constant over the course of the voyage in the revised circadian watch bill, it deteriorated over time in the traditional watch bill.

Figure 6.

Means and standard deviations of PVT reaction times in the traditional (red) and the circadian watch bill (blue) over the course of the 7-day voyages.

Discussion

To our knowledge, this is the first study comparing two two-section watch bills in an experimental cross-over design with two similar vessels operating at the same time in the same area, and each vessel running both watch bills. In this way, workload and weather conditions as well as potential differences between crews could be controlled. Thus, differences between watch bill conditions in subjective sleepiness, fatigue and in vigilance performance can be attributed with high certainty to the different on- and off-duty times of the sailors.

We did not identify any statistically significant differences between watch bills in terms of subjective sleepiness and fatigue. With a smaller proportion of highly fatigued or sleep-prone participants and a stronger decrease in ESS scores from first to second voyage in the circadian watch bill condition, there are some indications that that a 7-5-5-7 watch starting at 3 a.m. may be preferable over the traditional 6/6.

Despite the absence of clear subjective effects, however, we found a difference in psychomotor vigilance performance between the revised circadian and traditional watch bill. Specifically, PVT reaction times in the revised circadian watch bill remained stable. In contrast, reaction times in the traditional watch bill showed a significant increase with increasing duration of the voyage, with the most noticeable increase from day 6 to day 7. In general, our findings agree with the modeled predictions of Paul and Love (2021). In terms of performance, the 7-5-5-7 watch bill is indeed preferable over the 6/6 schedule. Contrary to Paul and Love, however, the 7-5-5-7 schedule did not only produce smaller performance decrements than 6/6, but no performance decrements at all. The reason for this may be the more evenly shared nighttime sleep opportunities in our study with a watch turnover time at 3 a.m. instead of 0:30 a.m. in Paul and Love. In fact, the performance trend of our circadian watch bill more closely resembles the modeled effectiveness of another two-section watch bill reported by Paul and Love (2021), the 8-4-4-8 starting at 4 a.m. Another discrepancy between modeled and observed performance pertains to the amount and timing of decrements in the 6/6 watch bill. These decrements occur earlier and stronger in the Paul and Love model and level off after 4-5 days, while they gradually built up in our data and increased after 6 days.

Overall, we found that the modelled predictions of performance in different watch bills could correctly indicate which one would be preferable. However, the actual level and dynamics of performance over time, which shows the importance of performance assessments in the field, were not captured in the models. The list of possible reasons for this is long and beyond the scope of this paper. But considering the differences between watch bills in sleepiness, fatigue and performance reported here, the answer to the question “How much can we improve two-section watch bills?” would be: just a little, at least for short voyages of up to 6 days. There were no clear benefits of 7-5-5-7 for subjective wellbeing, and sailors could limit performance decrements, for example by increased effort (Hockey, 2011), up to day 6 in the 6/6 watch bill. After six days at sea, compensation of sleep deprivation appears to be more difficult to sustain, so a different picture could emerge during longer voyages with two-section watch bills.

The rather short scope of the data collection with one-week voyages was due to the limited availability of the participating MCMVs and is clearly a shortcoming of our study. A limitation of this paper is the missing data on duration and timing of sleep episodes during the voyages. These data are currently being analyzed, and will complete the picture of the influence of the watch bills under study for the participating sailors.

Footnotes

ORCID iD

Stefan Röttger

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